Numerous adhesive bonding applications require tolerance of induced damage wherever an undetected debond can exist. The Federal Aviation Administration has mandated damage tolerance for all principal structural elements of transport category aircraft, as well as any repairs to these elements. Composite patch repairs of structural cracks have been shown to be extremely effective, yet they need a damage tolerant bonding mechanism in order to be certified for transport aircraft.
In practical terms, aircraft repairs should also be quick, durable, and relatively inexpensive. Some aircraft, primarily military, have used bonded composite patches to repair structural cracks. These patches have two beneficial effects. First, they provide an alternate load path. The patches are primarily unidirectional such that the fibers bridge the crack, transferring much of the load out of the plane of the damaged component. Second, they provide a virtual limit to the maximum crack opening. Both of these effects limit the stress intensity factor at the crack tip, which in turn limits the crack extension experienced in each load cycle. Patches are designed to keep the crack size well below its critical value during the remaining economic life of the aircraft. Boron/epoxy patches are used most often because of their high stiffness and the ability to monitor crack size through the patch using eddy current technology.
At present composite patches are generally limited to cracks small enough that they will not, over the course of several inspection periods, grow to a critical size for the largest expected lifetime load. In this way, even if the patch were somehow completely ineffective, the damage could be monitored and repaired via another method long before operational safety was jeopardized. Patches applied to small cracks could still economically extend the component""s lifetime, but could not be counted on to perform as primary structure.
Accordingly, it is an object of the present invention to provide a method of achieving structural redundancy and damage tolerance in any bonded joint.
It is a further object to provide such a method that is simple to use.
It is a further object to provide such a method that is economical.
It is a further object to provide such a method that can be applied to patches on aircraft components.